The ribosomal DNA (rDNA) from the yeast Saccharomyces cerevisiae is an interesting and versatile system for studying gene structure and expression, chromosome organization and chromosome replication in a simple eucaryote. A basic knowledge of the eucaryotic cellular regulatory mechanisms which govern gene expression and DNA replication is essential in understanding normal cell growth and alterations which occur in pathologic states such as infection, cancer, inherited metabolic errors, etc. The first section includes experiments which will examine the chromosomal location and the organization of the reiterated ribosomal RNA (rRNA) genes both genetically and biochemically. The discovery of a yeast strain which contains rDNA with an EcoRI restriction endonuclease pattern different from that usually found enables us to follow the segregation of rDNA in a genetic cross and to examine its linkage to other genetic markers. Biochemical experiments will include analysis of high molecular weight DNA containing rDNA by electron microscopy and restriction endonuclease digestion as well as the cloning and characterization of DNA adjacent to rDNA. The second section involves the control of expression and the fine structure of rDNA repeating units. We will characterize the pattern of transcription and the nature of the transcript from cloned rRNA genes in an Escherichia coli host. Also, we will compare rDNA repeating units from different strains and species of yeast by heteroduplex and restriction endonuclease analysis. The effect of any gross changes in rDNA structure on the expression of the rRNA genes will be studied. The final section of our experimental plan includes experiments to determine the timing of rDNA replication during the yeast cell cycle and to examine the fine structure of replicating chromosomes which contain rDNA.